Surface antigens that confer serotype specificity to Chlamydia trachomatis isolates are believed to function as protective antigens. The objective of this project is to identify the surface component(s) that possess these serotyping determiners with the rationale that they would be primary candidate antigen(s) for a subunit or live recombinant chlamydial vaccine. Monospecific polyclonal antisera and monoclonal antibodies have been raised against the chlamydial major outer membrane protein (MOMP) and characterized with respect to their specificity and function. The findings show that MOMP is the primary serotyping antigen of C. trachomatis possessing antigenic determinants of type, subspecies, and species specificity. These serologic properties were corroborated both by one- and two-dimensional peptide mapping of V8 protease and Alphaachymotrypsin-digested MOMP. Immunoelectron microscopy studies with MOMP monoclonal antibodies showed the MOMP type-specific epitope to be highly exposed on the chlamydial cell surface. Type-specific MOMP antibodies were capable of neutralizing in vitro infectivity. The type-specific epitope has been located on a 15Kd cyanogen bromide MOMP peptide fragment. Collaborative studies are being focused on the molecular cloning of the MOMP gene and developing chlamydial animal models in order to directly ascertain the role of the MOMP as a protective antigen.